Angioplasty has gained wide acceptance in recent years as an efficient and effective method for opening stenosis in the coronary arteries and in other parts of the vascular system. The most widely used form of angioplasty makes use of a dilatation catheter which has an inflatable balloon at its distal end. Using fluoroscopy, the physician guides the catheter through the vascular system until the balloon is positioned across the stenosis. The balloon is then inflated by supplying a fluid under pressure through an inflation lumen to the balloon. The inflation of the balloon causes stretching of the artery and pressing of the lesion into the artery wall to reestablish acceptable blood flow through the artery.
Intra-aortic balloon catheters have also gained wide acceptance in recent years. Intra-aortic balloon catheters are typically inserted into the aorta of the heart, often percutaneously, and then inflated and deflated out of phase with the natural pumping action of the heart. By doing so, the intra-aortic balloon catheters can supplement the natural pumping action of the heart. Both dilatation catheters and intra-aortic balloon catheters are balloon type catheters devices.
One important characteristic of balloon type catheters is the distal "profile", which is determined by the outer diameter of the distal end portion of the balloon when deflated. This outer diameter affects the ease and ability of the catheter to pass through a guide catheter, through the coronary arteries and/or across a tight lesion. Considerable effort has been spent in developing low profile balloon type catheters by minimizing the dimensions of the core or inner tube which extends through the balloon to its distal end, and by reducing the wall thickness, to the extent possible, of the balloon itself.
A complicating factor in minimizing the deflated profile of a catheter balloon is that the balloon membrane is often not distensible, i.e. it does not stretch or contract in response to changes in internal pressure. This is typically true for both dilatation catheters and intra-aortic balloon type catheters. Thus, the balloon membrane typically has a constant surface area regardless of whether the balloon is inflated or deflated. To reduce the outer diameter of the balloon catheter in its deflated condition, it is common to fold the balloon flat, so that two wings are brought together in some fashion, as by folding or wrapping, so as to reduce the overall diameter of the deflated balloon. In use, the inflation fluid that is applied to the folded balloon causes the flaps to unwrap so that the balloon can inflate to its full inflated state.
While it is desirable to minimize the profile of the catheter, it is also desirable to provide a large inflated outer diameter to the balloon. As the inflated outer diameter is made larger, the flaps of the balloon become relatively large relative to the core or inner tube of the catheter. The result is that it is often difficult to eliminate the interstitial space between the flaps when folded together or wrapped around the catheter.
Various methods and balloon configurations have been proposed in the prior art for providing a balloon type catheter that has the lowest profile possible when deflated and the largest possible diameter when inflated. One approach, which is suggested, for example, in U.S. Pat. No. 5,087,246 to Smith and in U.S. Pat. No. 5,147,302 to Euteneuer et al., is to provide a dilatation balloon having more than two flaps or wings (for example, three wings) such that when the flaps or wings are wrapped circumferentially, the distance that each flap extends around the catheter is reduced when compared with the two flap configuration. The ease with which such flaps fold is also enhanced when the number is increased, such that when the balloon is deflated and withdrawn through the guide catheter following a procedure, the balloon more readily returns to its wrapped condition. The result is a reduced deflated profile given the same inflated diameter.
Typically, the balloon flaps are formed during the manufacturing process of the catheter. U.S. Pat. No. 5,350,361 to Tsukashima et al. discloses a method for preparing a tri-fold balloon configuration. Tsukashima et al. initially impart the tri-fold configuration to the balloon by inflating the balloon in a longitudinal interstitial channel defined by three substantially cylindrical pins arranged in a pyramid-type stack. While the balloon is secured in this channel, negative pressure is applied to an inflation lumen of the balloon to deflate the balloon, thus providing the tri-fold configuration to the balloon.
The balloon may be "heat set" in the desired fold configuration so that the balloon returns to the fold configuration when the balloon is deflated. Tsukashima et al. suggest heating the creases defined by the three tri-fold flaps with a longitudinal heating element. This apparently softens the balloon material in the longitudinal creases, so that the same creases will tend to form whenever the balloon is deflated.
Once the flaps are formed and/or set in the balloon, it is common to manually fold the balloon flaps circumferentially around the catheter. The flaps are then typically held in place with a balloon protector. A balloon protector typically serves two functions. First, the balloon protector protects the balloon and the distal tip of the catheter from possible damage during shipping. Second, the balloon protector wraps the balloon tightly in its deflated condition to minimize the outer diameter of the balloon in its deflated state.
A typical balloon protector is applied to the distal end portion of the catheter prior to packaging and sterilization of the catheter. The sterilization process often involves exposing the catheter, with the balloon protector in place, to an elevated temperature for a predetermined time period. With certain balloon materials, such as polyolefin, the sterilization process causes the balloon to be "heat set" in the folded or wrapped condition in which it is held by the balloon protector. As a result, when the balloon protector is later removed, the balloon tends to remain in the tightly wrapped condition.
To further reduce the profile of the wrapped balloon, the balloon protector can be constructed to be radially compressible. This further reduces the interstitial space in the wrapped balloon particularly during the heat setting process. Thus, when a balloon material is used that exhibits heat set characteristics, the deflated balloon will tend to remain tightly compressed even after removal of the balloon protector.
While the prior art provides some improvement in the field of folding, wrapping and compressing balloon type catheters, there are still a number of limitations, some of which are discussed below. One limitation is that the prior art balloon flaps are typically manually folded over the catheter by an operator during the manufacturing process. This can be a relatively slow and tedious process, and the quality of the wrap is often dependent on the skill of the operator. Second, the balloon protector must typically be installed over the wrapped balloon, which can also be a slow and tedious process. It would be desirable, therefore, to provide an apparatus and method that helps form, wrap and compress the balloon flaps during the manufacturing process.
It would also be desirable to provide a tool that could be used by a physician during a medical procedure to reform the flaps and rewrap the balloon. It has been found that once a balloon has been inflated to relatively high inflation pressures, the balloon material can loose the heat set characteristics provided during the manufacturing process. For example, a tri-fold balloon that has been inflated to relatively high pressures (10-15 atm) may take on a pancake shape having, for example, only two flaps when deflated. Further, the balloon may not return to the original low profile wrap configuration. This can prohibit the crossing of additional lesions using the same catheter. To reach other lesions, the physician must often withdraw and discarded the catheter in favor of a new catheter that has a balloon that was tightly wrapped during the manufacturing process.